The present invention relates to a method of manufacturing a cable used in a device such as a computer, and more particularly to a method of manufacturing a multicore cable which needs accurate of electrical characteristics.
The multicore cable means a cable which has a plurality of signaling core wires in parallel. In most cases, the individual signaling core wire is covered with an inner insulating layer, and is formed together with a grounding core wire in such a manner that a shield is wound around them.
FIG. 2 shows a cross sectional view of a flat-type multicore cable as an example of the multicore cable used in a device such as a computer. The flat-type multicore cable is constituted by locating a plurality of cables in parallel. The individual cable is formed by paring a signaling core wire, which is covered with an inner insulating layer, with a grounding core wire, and then by winding a shield around the pair thus created. Since the flat-type multicore cable comprises a plurality of signaling core wires, grounding core wires and shields, it is one type of the above-described multicore cable. At the same time, the flat-type multicore cable, in which the plurality of cables are placed transversely in a line, has a flat configuration. In FIG. 2, a signaling core wire 6, which is covered with an inner insulating layer 5, is paired with a grounding core wire 4 provided along a side portion of the inner insulating layer, and a shield 3 of aluminum is wound around the pair, thus forming a shield layer. Moreover, a plurality of the cables, in any of which this shield layer is formed, are arranged in parallel and, with insulation between any two cables being maintained, the cables are fusion-welded using a jacket 2 made of a thermosetting resin.
In many cases, a component such as a connector is connected with one end of this flat-type multicore cable. This allows the flat-type multicore cable to be used in a state of being easily connected with or disconnected from an electronic appliance. JP-A-3-102783 discloses this flat-type multicore cable and a technique for connecting the component such as the connector with the flat-type multicore cable. The connecting steps, as shown in FIG. 4, are as follows: First, the jacket 2 and the shields 3 are cut and stripped, thereby exposing the inner insulating layers 5 and the grounding core wires 4. Second, being careful not to cut the signaling core wires 6, the inner insulating layers 5, which cover the signaling core wires 6, are cut. Third, being careful not to develop a short-circuit between an inner insulating layer 5 and a grounding core wire 4, the inner insulating layers 5 are stripped so as to expose the signaling core wires 6. Fourth, the grounding core wires 4 and the signaling core wires 6 are formed in such a manner as to fit a configuration of terminals 9 of a connector 8. Finally, the grounding core wires 4 and the signaling core wires 6 are connected with the terminals 9, thereby connecting the connector 8 with the flat-type multicore cable.
In the above-mentioned flat-type multicore cable 1, however, improvement has been made concerning the structure thereof and an insulator material of the inner insulating layer 5 so that the flat-type multicore cable 1 can respond to speeding-up of a signal transmission speed accompanied by an enhancement of machine cycle of a computer. In particular, the material of the inner insulating layer 5 is made closer to air so as to lower the permittivity thereof, thereby speeding up a transmission speed of a signal which transmits in the signaling core wire 6. As a result, the inner insulating layer 5 shown in FIG. 2 has become soft and has been found to be easily modified by the winding of the shield 3. This modification makes unstable a contact between the grounding core wire 4 and the shield 3, and especially when a subtle vibration is exerted on the cable, the effect of the poor contact becomes more apparent. This poor contact between the grounding core wire 4 and the shield 3 gives rise to a variation in characteristic impedance of the signaling core wire 6, and this variation in the characteristic impedance causes a failure to occur in the computer.
FIGS. 5A and 5B show variations in the characteristic impedance of the multicore cable. FIG. 5A is a graph representing the characteristic impedance when the cable itself is at rest. FIG. 5B is a graph representing the characteristic impedance when a vibration is exerted on the cable. It can be recognized that, as compared with the characteristic impedance shown in FIG. 5A, the characteristic impedance shown in FIG. 5B is varied more extensively under the influence of the vibration.
As a countermeasure to be taken against this, what can be considered, for example, is that gold with high conductivity is plated on a surface of the grounding core wire 4, thereby maintaining the electrical contact between them. However, this method is an expensive one because of the use of gold plating, and also was not successful in avoiding a problem in that the vibration makes imperfect the contact between the grounding core wire 4 and the shield 3. This poor contact between the grounding core wire 4 and the shield 3 has resulted in a drawback that the multicore cable lacks a reliability in a device such as a computer in which even a subtle variation in the characteristic impedance is not permitted.